RIYADH, Feb 24 (Reuters) - Saudi authorities arrested at least nine Saudi Shi'ite pilgrims after clashes in the holy city of Medina, Shi'ite and security sources said on Tuesday.

Saudi Arabia sees itself as the bastion of mainstream Sunni Islam and is worried about the rising influence of non-Arab Shi'ite power Iran in the region.

Jaafar al-Shaib, a leading figure among minority Saudi Shi'ites, said clashes occurred between Shi'ite pilgrims and morals police near a mosque that houses the tomb of Prophet Mohammad.

"Some 1,500 Shi'ite pilgrims gathered near the mosque for the commemoration of Prophet Mohammad's death," he said.

"Stick-wielding members of the morals police backed up by plainclothes policemen sought to disperse them."

Morals police often prevent pilgrims venerating tombs, seen as idolatry under the strict Saudi version of Islam.

Some pilgrims were injured in a stampede after police fired into the air to disperse the crowd, al-Shaib said, adding ambulances took some away. He said some shops owned by Shi'ites were attacked.

An Interior Ministry spokesman for security affairs described the incident as "a quarrel between visitors and worshippers".

"Now there is an investigation to establish motives and reasons," spokesman Mansour al-Turki said. He declined to confirm that the clash was between the morals police and Shi'ites.

He said nine people were taken in custody, but declined to give more details saying an official statement would be issued later.

A security source who asked not to be named because he is not authorised to talk to the media told Reuters seven Shi'ite pilgrims were injured in the resulting stampede and were taken to the city's King Fahd Hospital.